1. Field of the Invention
The present invention relates to a resin pallet with a spring structure and a method of producing the same, and more particularly to an inexpensive cushioning resin pallet with a spring structure, which is excellent in shock resistance, load capacity and durability, prolonged in service life, lightweight, and at the same time can be easily manufactured, and a method of producing the same.
2. Description of the Related Art
In general, wooden pallets are widely and frequently used for transshipping or conveying articles by forklift since they are lightweight, have surfaces with a friction coefficient large enough to make it difficult for the articles to slip thereon, and are capable of immediately absorbing water attached to surfaces thereof to thereby prevent the articles from being wet by the water. A wooden pallet of this kind has a body which is formed by arranging a plurality of upper and lower square wooden blocks in parallel with each other at predetermined space intervals, and connecting them by wooden plates with nails and screws. Hollow spaces defined by the square blocks and the wooden plates are used as fork insertion holes for inserting a fork of a forklift.
Although the wooden pallets most generally used today are advantageous in that they can be manufactured at low costs and have certain strengths, they are disadvantageous in that they are heavy in weight, and readily absorb water so that they are easily corroded, resulting in short service life. Further, they tend to be quickly soiled, which makes it difficult to employ them in food and medical industries. Still further, when they are stored in open yards, lower end portions thereof, which absorb and hence contain a lot of rain water, are decayed quickly.
On the other hand, plastic pallets have a complicated rib structure for reduction of weight thereof A plastic pallet is manufactured by injection-molding a top portion and a bottom portion of the pallet as separate pieces, and then fusion-bonding these portions. Although plastic pallets have longer service lives and are hard to be soiled, manufacturing costs thereof are much higher than those of wooden pallets, and they are low in durability and shock resistance.
More specifically, conventional plastic pallets are configured to have rib structures so as to impart strength thereto since they are made of plastic which is higher in specific gravity than wood. The rib portions of the pallets are thin, so that stress concentration can occur thereat. Therefore, although they exhibit longer service lives than wooden pallets, they can be deformed and cracked while they are used for a long time.
Further, pallets made of steel exhibit very high strengths but they are heavy in weight and corroded with rust.
The present invention has been made as a solution to the above problems, and an object thereof is to provide a resin pallet having a spring structure which prevents occurrence of stress concentration and permitting a long-term usage. Further, since the conventional resin pallets have complicated shapes, it is required to mold portions thereof by injection, injection compression, or extrusion compression. Moreover, it is necessary to mold the respective portions by using corresponding molding dies as well as carry out thermal fusion bonding and assembling operations. Therefore, another object of the present invention is to provide a resin pallet which can be molded with ease by a single molding apparatus and a single molding die, and have desired properties, particularly an excellent shock resistance and load capacity. Further, still another object of the present invention is to provide a resin pallet which has desired physical properties in shock resistance and load capacity, and at the same time includes a rib structure.
A resin pallet having a spring construction, according to the present invention, is a pallet of a three-dimensional structure with a predetermined bulk density and voids, the three dimensional structure being formed by contacting, entwining, and gathering adjacent ones of random loops or curls of continuous filaments and/or short filaments of a thermoplastic resin (hereinafter referred to as xe2x80x9cthe three-dimensional structurexe2x80x9d), characterized in that a single unit of the pallet is formed with high density portions having an increased bulk density at opposite ends of fork insertion holes of the single unit in a direction of length of the fork insertion holes and approximately at a central portion of the single unit in the direction, the high density portions extending to the opposite ends of the fork insertion holes in the direction of length of the fork insertion holes and opposite ends of the fork insertion holes in a direction of width thereof, and formed with flat portions having a bulk density not higher than those of the high density portions at least inner surfaces of the fork insertion holes or an outer periphery and the inner surfaces of the fork insertion holes being formed.
The present invention is also characterized in that, for instance, the continuous filaments and/or the short filaments are formed of a thermoplastic elastomer, that the continuous filaments and/or the short filaments have a diameter of 0.1 mm to 3.0 mm, preferably, 0.3 mm to 0.7 mm, or that a structure of the three-dimensional structure has a bulk density of 0.1 g/cm3 to 0.9 g/cm3. Further, an outer peripheral surface of the pallet including the inner surfaces of the fork insertion holes or the outer periphery of the fork insertion holes may be formed with flat portions having a bulk density not higher than that of the high density portions, or portions other than the high density portions and the flat portions may be formed as low density portions having a bulk density not higher than that of the flat portions.
If the continuous filaments and/or the short filaments forming the low density portions have a diameter of 0.1 mm to 3.0 mm, preferably 0.7 mm to 1.5 mm, the structure of the three-dimensional structure having a bulk density of 0.01 g/cm3 to 0.1 g/cm3, preferably 0.02 g/cm3 to 0.05 g/cm3, the weight of the pallet can be further reduced.
Further, when portions except the flat portions are formed with high density portions, by arranging two kinds of groups of injection ports for high density portions and low density portions, the molding of the pallet is made easier.
Further, the method according to the present invention is a method of producing a resin pallet by melt-extruding a thermoplastic resin and/or a thermoplastic elastomer into a plurality of filaments, and contacting, entwining and gathering adjacent ones of random loops or curls of continuous filaments, thereby forming the filaments into a pallet having a three-dimensional structure with a predetermined bulk density and voids, characterized by comprising the steps of forming high density portions and flat portions, or the high density portions and the flat portions as well as low density portions in a direction of width of the fork insertion holes by changing a take-off speed for taking off the fusion-extruded continuous filaments; extrusion-molding the fork insertion holes by using cores; and extruding, taking off, and forming high density portions and flat portions, or the high density portions and the flat portions as well as low density portions in a direction of length of the fork insertion holes, by changing a density of injection ports, on a portion-by-portion basis.
When the fusion-extruded material is taken off, in the direction of length of the fork insertion holes, the take-off speed is made faster in a manner corresponding to the length of the single unit than when the low density portions in the direction of length of the fork insertion holes are formed, and cutting the fusion-extruded material in a direction of width of the fork insertion holes to the length of each the single unit of the pallet. By this step, it is possible to continuously produce units of the resin pallet according to the present invention.